The use of Li-ion batteries (LIBs) as rechargeable power sources represents a promising technology for use in consumer electronics and automobiles. However, there are substantial technical challenges to the use of LIBs for automobile applications.
LIBs typically use lithium metal oxides such as LiCoO2 as the cathode; carbon or graphite as the anode; and a lithium salt such as LiPF6 in an organic solvent (e.g., organic carbonates) as the electrolyte. Since its commercialization, the capacity of LIBs has increased about 1.7 times due to improvements in battery structure, and anode or cathode materials. The capacity of the LIBs has been improved typically by increasing the amount of the active materials in the cathode, and anode, and by decreasing the thickness of the current collector, separator, and cell casing. For example, LIB capacity has improved by utilizing new cathode materials, such as layered Li[NixCOyMnz]O2 and related materials. Use of such new materials has provided about 9 to about 25% increase in the total mAh/g capacity over commercial cells; but this is still insufficient to satisfy the requirements of plug-in hybrid electric vehicles (PHEVs) or electric vehicles (EVs).
In addition to cathode materials, improved anode materials have also been investigated. Anode materials for LIBs typically fall into one of two types of materials: intercalation materials and alloy-forming materials. Graphite falls in the first category and allows intercalation of Li ions into its carbon layers for storage of lithium. Graphite exhibits good charge/discharge cycle stability, but low capacity. The theoretical capacity of graphite is 372 mAh/g based on a theoretical Li-to-C ratio (Li:C) of about 1:6 (i.e., LiC6).
Alloy-forming materials include, but are not limited to, Si, Sn, Pb, Al, Au, Pt, Zn, Cd, Ag, and Mg, can be used as alternatives to graphite. These materials store Li by forming alloys with Li. Si is one of the most attractive because of its relatively low discharge potential, the theoretical capacity (about 4200 mAh/g based on Li4.4S) and significant natural reserve (Si is the second most abundant element on earth). The disadvantage of alloy-forming materials such as Si is that the capacity fades rapidly due to very large volume expansions upon alloy formation. The large expansion and following contraction can cause disruption (e.g., pulverization) of the electrode and loss of electric contact between electrode materials limiting the cycle stability of these anode materials. For example, Si may undergo up to 400% volume change during the alloying and de-alloying process. Bulk Si is also not desirable as anode material because of a relatively low electrical conductivity, which can reduce the capacity of the LIBs.
Further improvement of LIBs require the development of new anode materials with desired properties.